Lousy displays, as those of you who've flown with them seem to agree. I have no idea, having never dealt with them.
However, this saved the aforementioned couple of million $ per year in fuel and reduces maintenance costs and failure rates, which speeding unscheduled repair when needed.
All these aircraft, as was said, have lots of cycles left and operate in short-haul environments where fuel burn is much less a factor than is dispatch reliability and low capital cost.
Delta does march to a different drummer from a fleet perspective. So, far, it works exceedingly well. They're consistently making money where other mainline carriers struggle. The 717's are an excellent case in point, as is this decision.